The classical production of inorganic pigments comprises, in general, the steps of homogenizing suitable raw materials, transferring the mixture into chamotte crucibles, annealing the raw-material mixture, breaking the incandescent material, single-stage or multi-stage wet grinding to coloring-matter fineness and drying. With a view to producing the raw materials, additional steps may be necessary, for example a precipitating process. The production consequently requires a high degree of technical effort which increases considerably with diminishing grain diameter. Pigments having a grain diameter in the range from a few microns down to submicron size are no longer obtainable in practice by means of the conventional comminution techniques. In addition, the risk then exists of influencing the color of the pigments as a result of attrition of grinding aids and mills that are employed.
The production of ceramic decorative colors and enamels has hitherto necessitated a joint grinding, ordinarily wet grinding, of a colored pigment with a flux, a glass frit for example, in order to achieve the fineness that is required for the screen printing and to enable good development of color in the course of decoration firing. Accordingly an objective of the invention consists in making available colored pigments for ceramic applications in such a form and granulation that a simple dry mixing may take the place of a grinding process.
The pigments obtained by conventional processes exhibit an irregular shape having fractured edges. Pigments of this type cause problems, even if the average grain diameter is sufficiently small and the grain-size range is sufficiently narrow, in the course of the spinning of materials colored with them. For instance, in the course of melt spinning, corrosion phenomena occur on the spinnerets or other faults arise, for example as a result of blocking of the spinnerets. Hitherto such problems have limited the use of conventional pigments of non-spherical shape for the production of spin-dyed fibers.
Pigments that are as finely divided as possible, preferably those which are intended in addition to exhibit a very narrow grain-size range, are particularly significant in all fields where the pigmented materials find application in the form of very thin decorative coatings, foils or fibers. An interest also exists in substantially spherical pigments, in order to avoid application problems caused by the hitherto irregular shape of the pigment particles.
From ES Patent 2 074 399 a process is known for the production of spherical, finely divided pigments on the basis of zirconium silicate. In this process a mixture consisting of a zirconium alkoxide such as Zr(O--n--C.sub.3 H.sub.7).sub.4 and a silicon alkoxide such as Si(OC.sub.2 H.sub.5).sub.4 is prehydrolyzed by means of moist air. After addition of vanadyl chloride--molar ratio V/Zr 0.02 to 0.04--and conversion of the precondensed mixture thus obtained into an aerosol, condensation is completed by means of moist air. The amorphous powder is then calcined for 24 h at 1500.degree. C. Disadvantages of this process are the long reaction-times for the prehydrolysis and calcination. In addition, as has been determined on the basis of the investigation of spinel pigments and ZrO.sub.2 pigments, in the course of calcination the initially spherical particles become caked together, necessitating a subsequent grinding process and therefore impairing the spherical shape of the pigment particles.
Ceramic spherical particles in the range from a few microns down to submicron size are available by means of the so-called spray-pyrolysis technique. With this technique an aerosol consisting of a solution or suspension of substances from which the fine powder to be produced can be formed is introduced into a heatable reactor, whereby the solvent or suspending agent evaporates and solid residual particles are formed which can in turn be converted by a decomposition reaction or another chemical reaction into the desired product.
Using a solution containing Mg nitrate and Al nitrate, Mg--Al spinel can be produced (Ceramic International Vol. 8 (1982) 17-21) by spray-pyrolysis technology in the form of hollow spheres having a particle diameter of 1 to 10 .mu.m. Through the use of a combustible gas for the purpose of atomizing the solution, it is possible to obtain MgAl.sub.2 O.sub.4 and also other colorless oxide-ceramic powders on the basis of Mg, Y, Al and Zr having a particle size in the range from 0.1 to 3 .mu.m (EP-B 0 371 211). The exclusive or concomitant use of metal compounds of color-imparting elements in connection with the spray-pyrolysis technique for the purpose of obtaining a colored oxide or mixed oxide and the use thereof as colored pigment is neither mentioned nor suggested in either of the two aforementioned documents.
According to EP-A 0 681 989, with a view to producing a multi-element metal oxide by way of precursor for high-temperature superconductivity ceramics by spray pyrolysis, the aerosol is introduced into an independently operated oxyhydrogen flame, whereby a lower flame temperature, namely 800 to 1100.degree. C., is adjustable and hence the mixed oxides are obtainable not in the high-temperature modifications that are less suitable for the stated purpose but in other modifications. A further modification of the spray-pyrolysis process with a view to producing metal-oxide powders for heavy-duty ceramics is taught by DE-OS 43 07 333. According to this patent a micro-emulsion consisting of an organic phase and an aqueous solution dispersed therein of the metal compounds is fed in aerosol form to the pyrolysis reactor. Even though color-imparting elements such as V, Cr, Mn, Fe, Co, Ni and Cu are also named among the elements as sources for the metal-oxide powders, in this case it is a question, as the example of DE-OS 43 07 333 also shows, of elements that are contained in a small quantity in a matrix which in itself is colorless. Suggestions with regard to employing compounds of color-imparting elements on their own or in combination with other compounds in a solution to be fed in the form of an aerosol to the pyrolysis reactor in such a quantity as to result in powders of intense color and with regard to using the latter as a colored pigment cannot be inferred from the two last-named documents.